Tidal power harnesses the natural energy produced by the periodic rise and fall of the sea. These tides are created by the rotation of the Earth in the presence of the gravitational fields of the Sun and Moon.
Various methods may be employed to convert the energy of the tides into useful power. These methods broadly fall into two categories: tidal stream systems and tidal barrages.
Tidal stream systems operate in a similar manner to wind turbines and usually consist of a turbine which is rotated by the tidal current.
With a tidal barrage, water is allowed to flow into the area behind the barrage (for example, an estuary) through sluice gates during the flood tide. At high tide, the sluice gates are closed. Since the sea level falls during ebb tide, a head of water is created behind the barrage. Once the head of stored water is of sufficient height, the sluice gates are opened and the stored water is directed to flow through turbines housed within the barrage, thus converting the potential energy stored in the water into useful power.
A tidal barrage is in use on the Rance river in France. The Rance tidal barrage use 24 turbines, each capable of outputting 10 Megawatts of power. The turbines are low-head bulb turbines which capture energy from the 8 metre tidal range of the river using a 22.5 km2 basin.
FIG. 1 shows a cross-section through a tidal barrage as used on the Rance river.
The tidal barrage separates an upstream side 102 and a downstream side 104. A passage is formed through the barrage in which a bulb turbine 106 is positioned. The flow of water through the passage and turbine 106 is controlled by first and second turbine gates 108, 110 located at either end of the passage.
The turbine 106 comprises a generator 112 at an upstream end of the turbine 106. The generator 112 is positioned centrally in the turbine 106 and water is forced to flow around the outside of the generator 112 over a set of stationary guide vanes 114 to a rotor 116. The rotor 116 is rotatably coupled to the generator 112 and comprises a plurality of blades. The blades of the rotor 116 have a hydrofoil cross-section which creates torque and rotates the rotor 116 when water flows past the rotor 116. This turns the generator 112 and thus produces useful power.
The turbines used in the Rance tidal barrage were intended for bidirectional operation (i.e. dual generation where power is generated on both ebb and flood tides). However, the low efficiency of the turbines during flood tide has meant that the turbines have only been used for ebb generation.
An electrical grid transmits the electrical energy from the generator 112 of the turbine 106 to electricity customers. The electrical grid provides an electrical load to the turbine 106 and generator 112 which imposes a torque on the rotor 116 of the generator 112. The torque controls the rotation of the rotor 116 and prevents overspeeding.
In the event of a disconnection from the grid (a loss-of-grid event), the grid load is lost, resulting in a loss of torque on the generator 112. This loss of torque can lead to a large acceleration of the turbine 106 which is limited only by hydrofoil drag. Such overspeeding can cause damage to the turbine 106 and generator 112, particularly bearings, drive train components and the rotor blades themselves, and also to the surrounding structure.
The turbine gates 108, 110 may be closed to prevent overspeeding of the turbine 106 and generator 112 during a loss-of-grid event. Typically, sluice gates (not shown) in the barrage are opened to enable water to bypass the turbines 106 and to flow through the barrage. This prevents the head of water from exceeding the safe working limits of the barrage and avoids overtopping where the water overturns the barrage structure.
With a dual generation barrage the differential head of water against the barrage is low enough that entry, through duct and exit losses are substantial and a large number of turbines are required to provide a highly porous structure in which these losses are minimised. The high percentage of turbine swept area results in there being no additional area available for the provision of sluice gates. The barrage is designed such that structure is capable of withstanding a differential head substantially less than the full tidal range of the estuary. Accordingly, closure of the gates across each turbine may result in an unacceptably large head building across the barrage. This head of water may exceed the safe working limits of the barrage and thus is undesirable.
An alternative strategy frequently used is to adjust the pitch of the rotor blades, so as to reduce the speed of the turbines. However, this results in a sudden increase in flow through the barrage, causing a wave. The wave may proceed up through the estuary and can impact upon the inland environment. Furthermore, the installation will not be able to resume generation until the next tidal cycle.
The present invention seeks to provide a turbine array which mitigates against a loss-of-grid event whilst addressing some or all of the above identified problems.